Gas chromatographic mass spectrometers (GC/MS) are made up of a gas chromatographic apparatus (GC) and a mass spectrometer (MS). Thus, first respective substances to be measured included in the substance (sample), which is the object of analysis, are separated along the time axis using a gas chromatographic apparatus. Next, the respective substances to be measured separated along the time axis are measured using a mass spectrometer (MS), so that the substances to be measured are separated based on the mass number, and thus detected. Measurement is repeated at short time intervals, so that a number of mass spectra having a mass number along the lateral axis and an ion intensity along the longitudinal axis are prepared. In addition, the peaks of a certain mass number in the number of mass spectra are focused, and the focused peaks are aligned along the time axis, so that a mass chromatogram is prepared. Furthermore, all of the peaks in one mass spectrum are integrated and aligned along the time axis, and thus a total ion chromatogram is prepared.
In addition, the retention time for the substance to be measured is determined on the basis of the peak in the total ion chromatogram. Furthermore, in order to calculate the retention index of the substance to be measured, n-alkane of different carbon numbers and the substance to be measured are introduced in the GC/MS, and as a result of measurement, the relationship between the peaks of the n-alkane of different carbon numbers and the peak of the substance to be measured in the total ion chromatogram is found, and thus, the retention index of the substance to be measured is calculated.
Here, unlike the wavelength in spectrometry and the mass number in mass spectrometry, the “retention time” in gas chromatographic apparatuses is not a value that is uniquely determined on the basis of the properties of the substance to be measured, but affected by a number of factors, such as the type, dimensions and temperature of the column, the type, pressure and flow amount of the carrier gas, and the type of apparatus. Meanwhile, the “retention index” is an index depending on the retention time for a predetermined standard substance (generally n-alkane), and has a numeric value which is not affected by the difference in the GC conditions, the column maker, the length, the inner diameter and the film thickness. That is to say, one numeric value is given to each standard substance, and the carbon number×100 is given to n-alkane, so that 1000 is given to n-C10 alkane and 1100 is given to n-C11 alkane, for example.
Thus, in the case where the peak of the substance to be measured in the total ion chromatogram is between the peak of the n-Cn alkane and the peak of the n-Cn+1 alkane, for example, the peak of the n-Cn alkane, the peak of the n-Cn+1 alkane and the peak of the substance to be measured are used to calculate the retention index (ix) of the substance to be measured using the following formula (1) or (2) (see FIG. 8).
(i) At time of analysis at constant temperatureix=100·(tx−tn)/(tn+1−tn)+n  (1)(ii) At time of analysis while temperature risingix=100·(log tx′−log tn′)/(log tn+1′−log tn′)+n  (2)
Here, in+1 is the retention index of the n-Cn+1 alkane, in is the retention index of the n-Cn alkane, tn+1 is the retention time for the n-Cn+1 alkane, tn is the retention time for the n-Cn alkane, tx is the retention time for the substance to be measured, tn+1′ is the corrected retention time for the n-Cn+1 alkane, tn′ is the corrected retention time for the n-Cn alkane, and tx′ is the corrected retention time for the substance to be measured.
When the retention index (ix) of the substance to be measured is gained in this manner, whether or not the mass spectra of the substances registered in a database having holding indices in a certain range including the retention index (ix) of the substance to be measured include mass spectra which are the same or similar to the mass spectrum of the substance to be measured can be determined. Here, reverse searching methods, for example, can be cited as methods for determination. Reverse searching methods are searching methods for calculating the probability of the substance to be measured being a registered substance by calculating to what extent the relative intensity of the peak of the main fragment ions in the mass spectra of the registered substances is similar to the mass spectrum of the substance to be measured.
Conversely, in the case where the retention index (ix) of the substance to be measured is gained in advance, the peak of the n-Cn alkane and the peak of the n-Cn+1 alkane in the total ion chromatogram can be used to calculate the retention time (ix) for the substance to be measured using the formula (1′) or (2′). Thus, the retention time (ix) of the substance to be measured is gained, and the substance to be measured is easy to adopt.
(i) At time of analysis at constant temperaturetx={(ix/100)−n}·(tn+1−tn)+tn  (1′)(ii) At time of analysis when temperature risinglog tx′={(ix/100)−n}·(log tn+1′−log tn′)+log tn′  (2′)Patent Document 1: Japanese Unexamined Patent Publication 2003-139755